I. Field of the Invention
The present invention relates to wireless telecommunications. More particularly, the present invention relates to a novel and improved method and apparatus for generating a reduced peak-to-average amplitude high data rate channel using of a set of lower rate channels.
II. Description of the Related Art
The IS-95 standard defines an over the air interface for providing more efficient and robust cellular telephone service using code division multiple access (CDMA) technology. CDMA technology allows multiple channels to be established within the same radio frequency (RF) electromagnetic spectrum by modulation of the data to be transmitted with one or more pseudorandom noise (PN) codes. FIG. 1 provides a highly simplified illustration of a cellular telephone system configured in accordance with the use of IS-95. Mobile telephones 10 (also referred to as wireless terminals) communicate with base stations 12 via CDMA modulated RF signals, and base station controllers 14 provide call control functionality that allows mobile telephony to take place. Mobile switching center (MSC) 16 provides call routing and switching functionality to public switch telephone network (PSTN) 18.
Conducting communications within the same RF band allows adjacent base stations to use the same RF spectrum, which increases the efficiency with which the available bandwidth is used. Other cellular standards typically require adjacent base station to use different RF spectrum. Using the same RF band also facilities performing "soft handoff," which is a more robust method of transitioning a wireless terminal (typically a cellular telephone) between the coverage area of two or more base stations. Soft handoff is the state of simultaneously interfacing the wireless terminal with the two or more base stations 12, which increases the likelihood that at least one interface will be maintained at all times during the transition. Soft handoff can be contrasted to the hard handoff employed by most other cellular telephone systems where the interface with the first base station is terminated before the interface with the second base station is established.
Another benefit of using the same RF band to conduct communications is that the same RF equipment may be used to transmit a set of lower rate channels. This allows the same RF equipment to be used to generate a higher rate channel formed by multiplexing the higher multiplexed over the set of lower rate channels. Transmitting multiple channel using the same RF equipment is in contrast to frequency division and time division multiple access (FDMA and TDMA) systems, which generally cannot simultaneously transmit multiple channels using the same RF equipment since the channels are frequency divided to a greater degree than in a CDMA system. This ability to transmit higher rate channels using the same RF equipment has become another important advantage of IS-95, as the world wide web, video conferencing, and other networking technologies has created a need for such higher rate channels.
Although higher rate channels are more easily formed within a CDMA system by channel bundling, the overall system performance resulting from this bundling is not optimal. This is because summing multiple channels creates a higher peak-to-average amplitude waveform than that of a lower rate serial channel. For example, for a serial channel the amplitude of the data waveform is either +1 to -1, in accordance with the BPSK data modulation employed by IS-95. Thus, the peak-to-average ratio is essentially that of a sin wave. For a higher rate channel that sums four lower rate channels, the amplitude of the waveform can be +4, -4, +2, -2 and 0. Thus, the peak-to-average amplitude of the bundled channel would be significantly greater than a sin wave, and therefore significantly higher than the non-bundled channel.
An increased peak-to-average amplitude places greater demands on the transmit amplifier of a system, and can reduce the maximum data rate or maximum range at which a system can operate. This is due to several factors, the most important of which is that the average data rate is dependent on the average transmit and receive power, and a higher peak-to-average amplitude waveform requires a greater maximum transmit power to sustain a given average transmit power. Therefore, a larger, and more expensive, transmit amplifier is required to provide the same performance for a higher peak-to-average waveform. Nonetheless, it is highly desirable to generate a higher data rate channel in a CDMA by bundling a set of lower rate channels. Thus, there is a need for a method an apparatus for reducing the peak-to-average transmit amplitude ratio for a set of bundled lower rate CDMA channels.